1. Field of the Invention
The invention relates to a FM modulation system using a quadrature modulator, and particularly, to a FM modulation system using a quadrature modulator, wherein a FM modulator and a system for taking out an output signal therefrom can be dispensed with through processing of a baseband signal and subsequent operation of quadrature modulation thereof in coping with a dual mode communications system incorporating a FM (frequency modulation) method in combination with CDMA (code division multiple access) techniques.
2. Description of the Related Art
The CDMA techniques used by conventional communications servers in Japan always incorporate both analog TACS (total access communication system) and a CDMA method, and since respective modulation methods differ from each other, the TACS adopts a FM modulation while the CDMA method adopts quadrature modulation.
FIG. 4 is a block diagram showing the construction of a conventional dual mode communications system incorporating both the TACS and CDMA method.
As shown in FIG. 4, in the TACS adopting frequency modulation, a modulating signal fm is inputted to a FM modulator 1 for frequency modulation, and subsequently, after converting frequency through multiplication thereof by a local signal LA at a multiplier (mixer) 2, a modulated wave is taken out by switching a change-over device 3.
Meanwhile, in the CDMA method, a baseband signal ba and the local signal LA are inputted to a quadrature modulator 4 for quadrature modulation ,and a modulated wave is taken out by switching the change-over device 3.
Thus, with the conventional dual mode communications system incorporating both the TACS and CDMA method, the TACS employs the FM modulator 1 wherein an output signal from an oscillator (not shown) is frequency modulated in a frequency band in the range of about 40 to 50 MHz by the modulating signal fm such as an audio signal, and further converted to the order of 800 MHz in frequency by the multiplier (mixer) 2.
Accordingly, the oscillator is a must for the FM modulator 1, and has a frequency band in the range of about 40 to 50 MHz. Since the output signal from the oscillator which is frequency modulated in the frequency band in the range of about 40 to 50 MHz by the modulating signal fm, always has side bands generated at the sides thereof, and is further converted to the order of 800 MHz in frequency after multiplication by the local signal LA at the multiplier 2, removal of the side bands is called for.
For removal of the side bands, however, a filter having a characteristic of sharp frequency cut will be called for, but it is technically very difficult and very high in cost to manufacture such a filter having the characteristic of sharp frequency cut.
Although it is also conceivable as an alternative to lower the oscillation frequency of the local signal, this will entail necessity of increasing the oscillation frequency of the FM modulator, and as it is difficult to manufacture a FM modulator at high frequency, an increase in cost will result.